For conventional charge pump circuits, the pump driving capability can degrade due to low power supply voltage (VDD), low temperature, or high threshold voltage (Vt) of Metal-Oxide-Semiconductor (MOS) transistors with thick gate oxide. More specifically, these factors can adversely affect a charge pump doubler circuit in the charge pump from doubling its input voltage range for its output. If the charge pump doubler is unable to generate voltage output high enough, e.g., close to two times of the input voltage such as VDD in some applications, it can suffer from low efficiency, e.g., from current flowing back to drop the output voltage. Therefore, new circuits and methods are desired to solve the above problems.